For the solid lubricants which are, in use, embedded in pores or grooves formed in a bearing sliding surface, it is desired that such solid lubricants can be easily introduced to a bearing sliding surface through sliding contact with a mating member (such as rotating shaft) for forming there a strong solid lubricant film, and that the formed film be capable of enduring repeated frictional contact for a long time and also exhibit an excellent self-repairing ability should a break be produced in the film. As such solid lubricants, there are popularly used those containing graphite as a main component, thanks to the following advantageous properties of the graphite. The graphite, because of its lamellar crystal structure, exhibits a large resistivity in the direction of load applied thereto, but exhibits a small resistivity in the sliding direction. Furthermore, the graphite is soft and able to maintain good lubrication performance over a broad temperature range of from an ordinary temperature to a high temperature.
The solid lubricant containing graphite as a main component, however, is not only rather insufficient in film forming capability, but also unsatisfactory in film life when repeatedly exposed to frictional contact, so that it has the problem that the sliding members (bearings) applied with the lubricant film are subject to restrictions regarding their use conditions and can hardly stand use under high-load conditions.
As other solid lubricants, there has been proposed a solid lubricant comprising lead, a polytetrafluoroethylene resin, a polyolefinic resin and a wax. (See, for instance, Patent Document 1.) This solid lubricant can meet substantially all of the conditions required for the solid lubricants; it is very low in coefficient of friction under the high-load conditions, has excellent film-forming capability, can provide a film with a long service life, and exhibits a high self-repairing ability for the film. In line with the recent trend toward larger size and higher performance of the machines and apparatus, however, further improvement of loading endurance and improvement of friction and wear properties are required of the sliding members having a solid lubricant embedded therein.
From another viewpoint, it is noted in recent years that the tendency of the development of material is obliged to give considerations to the environmental problems, and there is posed the problem that although lead is a material that makes it possible to meet all of the requirements for the solid lubricants, its use is still limited as lead is an environmental loading material.
In view of the above circumstances, the present applicant had previously proposed a solid lubricant comprising a polyethylene resin, a hydrocarbon-based wax and melamine cyanurate. (See, for instance, Patent Document 2.) This solid lubricant is free of lead, which is an environmental loading material, in its composition and exhibits the friction and wear properties identical to or better than those of the conventional lead-containing solid lubricants, and this has presented a certain solution to the above problem. In the ensuing experiments, however, there has been raised a new problem. This solid lubricant has an “island-and-sea” structure in which a sea phase (continuous phase) is formed by a polyethylene resin and a hydrocarbon-based wax, and an insular phase (dispersion phase) of melamine cyanurate is dispersed in the sea phase. In a sliding test conducted with this solid lubricant embedded in pores formed in the body of a sliding member, there took place an expansion and softening of the solid lubricant under the influence of frictional heat generated at the sliding surface, and the solid lubricant was caused to make a plastic flow as a result of subjection to repeated frictional contact in this state, consequently causing loss of part of the solid lubricant.    Patent Document 1: Japanese Patent Publication (KOKOKU) No.    Patent Document 2: International Publication WO2004/046285